1. Field of the Invention
This invention is directed to methods and devices for detecting a loss of signal in an optical transmission system, and more particularly, to detection of a loss of signal and the inputs of bidirectional optical amplifiers.
2. Background Art
In an optical transmission system, a loss of input signal alarm is an important tool for determining that an optical connector has been disconnected, a cable has been broken, removed, or introduces a high loss.
Moreover, in optical amplified systems, the reflection of a significant portion of the light leaving via a given fiber may cause problems with detection of the loss of the input signal on that fiber. The outgoing light, that is then reflected, could be amplified signal and amplified spontaneous emission (ASE), in the case of a bidirectional system, or could be just ASE, in the case of a unidirectional system. Or, the outgoing light could be a combination of signals and ASE from both directions in the case where there are more complex optical path reflections. If the reflected outgoing light could be distinguished from the desired input signal, then appropriate alarms or control actions could be initiated.
In bidirectional optical amplifier applications, whenever both connectors are open with the pump laser in both directions being "on", reflections can cause Q-switching and oscillations, despite the optical isolators that may be present. The oscillation path can involve more than one optical amplifier in the system and be quite complex. Prevention of this condition is desirable, as this phenomena can cause damage to the amplifier.
If a fiber is accidentally disconnected from one particular input of the amplifier, the amplifier should be shut off in that direction to minimize the risk of self-oscillation and Q-switching. In addition, shutting off the laser pump must occur very fast, before the other fiber can be disconnected.
For this reason, it is advantageous for a bidirectional amplifier to have a fast method of detecting a loss of signal (LOS) due to a disconnected fiber, in order that the amplifier can be shut off quickly in the corresponding direction.
Measurement of the strength of reflections is presently done with an optical time domain reflectometer (OTDR) that sends strong short pluses of light down a fiber and measures the signal returned to determined LOS conditions from sudden increases in reflected power levels. This is an accurate method, but the OTDR is a relatively large and expensive piece of test equipment that can not be easily used while there is traffic on the fiber. Also, as an OTDR is a separate device from the amplifier, there is no direct means of shutting off the amplifier in a particular direction when the reflection condition is detected.
Another prior art method for detecting reflections is to measure the amount of DC light reflected back via a four port coupler. However, this method cannot be used in bidirectional systems.
Techniques such as disclosed in U.S. patent application Ser. No. 08/588,776 (O'Sullivan et al.) filed Jan. 18, 1996 and assigned to Northern Telecom Limited) can be used to measure reflected power on an in-service link at the amplifier site. This method however, can take in the order of a few seconds to provide accurate results following a change in conditions at an input port, leaving enough time for the second input port to be disconnected following the first, resulting in above mentioned undesired mode of operation.
There is a need to provide a fast means for detecting LOS in a transmission system equipped with bidirectional optical amplifiers, irrespective if a data signal is present or absent.